Currently MEMS is the leading technology for implementing all-optical cross-connect switches for fiber optic telecommunication networks. Typically these devices work by collimating light from an array of input fibers and reflecting these light beams off an array of movable micromirrors, which then act to deflect each beam toward the appropriate output fiber. For large switches (more than 100 input and output fibers) the most popular configuration has the micromirrors arranged on a rectangular grid with their faces parallel to the substrate. The input light beams are directed at the micromirror array in the normal direction, and the mirrors tilt on two axes to deflect each input beam in the desired direction. Very often the deflected light beams are retro-reflected by an ordinary bulk mirror back onto other micromirrors in the array. These mirrors are then tilted so as to direct the light beams onto the appropriate fibers in the original fiber array. This way, any fiber can be cross connected to any other fiber in the array without having to partition the fibers into input and output groups and restricting cross-connections to between these groups only.
This so-called “3D MEMS” configuration has several drawbacks:                (1) Switching Speed: The mirrors are 100's of microns in diameter and fairly thick (to prevent warping). Their large size and mass give them a large moment of inertia, which limits their tilting speed to about 10 ms. Moreover this problem becomes exacerbated as the number ports in the switch increases, because this implies a smaller angular separation between ports. In turn, this means that beam spreading due to Gaussian optics and diffraction must be reduced, and only increasing the mirror diameters can do that.        (2) Control: Using a 1024×1024 switch as an example, a switch this size would have a 32×32 fiber array, and therefore the mirror tilt would have to be controlled to a precision considerably greater than {fraction (1/32)} of full deflection. This requirement only becomes more stringent as the switch size increases.        (3) Fabrication: A large mirror diameter requires that the mirrors be suspended high above the substrate so that there is sufficient clearance for the mirror to tilt to its maximum deflection (typically about 10°). These kinds of structures are difficult to fabricate with currently well-established micromachining techniques. Often, designs resort to pop-up structures that are fabricated with thin, surface micromachined structures that fold up into their final configuration. However this approach significantly complicates the device. Furthermore, making large diameter mirrors that are stiff and optically flat is difficult.        (4) Actuation: Arrays of tilt-mirrors are typically actuated electrostatically. However this type of actuation has a possible collapse instability (“snap-down”), which arises because the electrostatic force varies as the inverse square of the electrode separation, whereas the mechanical restoring force typically increases only linearly. To avoid this instability, the minimum electrode separation must be at least ⅓ of the initial gap between electrode and mirror. In turn, this means that the already large clearances required by large diameter mirrors must be three times larger still. Furthermore, the mirror's large moment of inertia will require large electrostatic and restoring forces, if it is to have reasonable switching times. In turn, this means that large driving voltages (˜100V) are needed.        
The conception of tilt-mirror optical switches is based on geometric optics, i.e., light rays from the input fiber are reflected to the desired output fiber, and the appropriate tilt angle for the micromirror are calculated from the geometric optics law “the angle of incidence equals the angle of reflection.” Alternatively, designs can be based on the more general theory of wave optics, where light is understood to be an electromagnetic wave rather than a geometric ray. In this picture, an optical switch element alters the amplitude and/or phase profile of the incoming wavefront so that the light wave then propagates toward the desired output fiber. In the case of the tilt-mirror switch, the tilted mirror induces in the wavefront a phase delay that varies linearly across the face of the mirror, and it is this linearly tapered phase profile that is responsible for redirecting the light wave propagation.
Instead of a continuous linear taper, one might look for a different phase profile that would steer the outgoing light beam. One example is the variable-blaze diffraction grating. On the other hand, the most obvious possibility is to simply replace the continuous linear phase profile of the tilt-mirror with a staircase approximation, which is the principle on which phased array antennas work. A tiling of small mirrors that move up and down in a piston-like fashion can generate such a staircase phase profile. This array is essentially a replacement for a single tilt-mirror, so an N×N switch would thus consists of 2N copies of this array, two for each input beam. This approach has several advantages over tilt-mirrors:                (1) Switching Speed: The much smaller diameter of these mirrors reduces their masses in two ways. First, the area of each mirror is much smaller. Second, the smaller mirror does not need to be as thick to maintain optical flatness. This drastic reduction in mass means that the mirror can be switched much faster—in about 10 μs.        (2) Control: Although these mirrors have about the same number of positions as the tilt-mirrors (16 vertical positions vs. 32 tilt-angles along a given axis), the precision with which these positions need to be controlled is much less. This scheme is more tolerant to positioning errors.        (3) Reliability: The performance of this device will degrade gradually if individual mirror elements fail. In contrast, almost any failure mode of the tilt-mirror will have a catastrophic impact on its performance.        (4) Fabrication: The vertical displacement of the mirrors will not need to exceed ½λ, which is 780 nm for the fiber optic communications C-band. Therefore the required clearances are much smaller, and the mirror structures can be fabricated using straightforward surface micromachining techniques.        (5) Actuation: These small clearances also allow smaller driving voltages.        
Despite these advantages, the phased array approach has one very large drawback, i.e., the huge number of mirrors that must be fabricated. Because of diffraction, the reflected beam has a small spreading angle. This limits the number of non-overlapping steering angles, and this number is a function of the number mirrors in the array. A micromirror array producing a 32×32 grid of distinct steering angles will need to have at least 100×100 elements. Therefore a 1000×1000 OXC switch based on 100×100 micromirror arrays will have 20 million mirrors. Even so, an array this size will have a diffraction efficiency of only about 80%, and a 99% efficient array would need to be at least 512×512 elements.
Not only does this many mirrors need to be fabricated with good yield, but the control circuitry must also be duplicated 20 million times as well. Even merely addressing and running control lines to this many mirrors is a formidable challenge.